Refers to the lively energy or spirit that makes someone or something feel alive and vibrant. Often used when describing a person's expression, a place's atmosphere, or the quality of a performance.
His face was full of vitality.
This painting is brimming with life.
生気 refers to the life force or vital spark within a person or living thing, whereas 活気 describes the lively atmosphere of a place or group.
Compound of 生 (life, living) and 気 (spirit, energy). The combination straightforwardly conveys the idea of life energy or vitality.